Engine piston and manufacture

ABSTRACT

An engine piston is manufactured by assembling an outer shell, comprising a crown and tubular side wall in which a ring groove region and skirt are defined, with a plate-like mounting member and bonding them together by brazing or welding. The mounting member is located within the tubular side wall displaced axially from the crown and is bonded near, but displaced radially from, its centre to the crown and at its periphery to the side wall at the end of the ring groove region. The mounting member carries gudgeon pin boss means facing away from the crown. The outer shell is formed by extrusion or the like that permits minimal wall thickness and the bonded structure is of light weight but great strength and stiffness, particularly in the ring groove region. A combustion bowl formed in the crown facilitates bonding to the mounting member and defines with the mounting member an annular cooling chamber adjacent the ring groove region and crown.

This invention relates to pistons for internal combustion engines and tothe manufacture thereof. The invention is particularly concerned withthe manufacture of a strong piston that is also light in weight andsuitable for cost-effective mass production for small capacity, highcompression engines.

Pistons for internal combustion engines for mass market automobiles aremanufactured in large numbers and subjected to cost constraints, whichin turn place limits on manufacturing processes. Such pistons areusually, but not necessarily, cast from a light metal alloy, typicallyaluminium based, and then subjected to a series of machining steps thatculminate in a precision component.

For heavy duty use, for example in compression ignition engines, it isknown to manufacture pistons of steel, usually forged, but such pistonshave tended to have a weight penalty, notwithstanding extensivemachining operations to remove extraneous metal, and have thus far beenrestricted to large capacity, low-revving engines found in trucks andthe like.

In recent times there has been a need to provide such compressionignition engines for use in smaller, automobile engines, where it isnecessary to run at higher speeds and with such high compressionpressures. Although steel is a material having suitable properties, andhas such strength that it could be used in relatively thin sections thatmitigate most if not all of the weight penalty, there is difficulty inmanufacturing a small one-piece piston that is capable of fulfillingsuch potential. In general such a piston has to be manufactured in steelby forging, with attendant limits to wall thicknesses and shapes thatlimit weight reduction.

It has been proposed to assemble or construct a steel piston fromseparately manufactured parts, as in U.S. Pat. No. 1,667,202 and U.S.Pat. No. 2,244,008. However the crown forms of the pistons shown thereinare relatively simple in structure and without an in-crown combustionbowl often required by modern engines. Even without such addedcomplexity, it is believed that the number of separate parts andassembly operations required are not conducive to providing a smallpiston capable of operating within a modern small engine in acost-effective manner.

Notwithstanding that a small piston for mass production is subjectedprincipally to constraints of cost, a larger piston for heavy dutyapplication is subjected principally to constraints resulting fromweight, so that the ability to produce a light weight piston costeffectively is not restricted in applicability. With this in mind, it isan object of the present invention to provide an engine piston ofassembled form that is capable of providing strength and light weight insimple form and a method of producing such a piston that is capable ofimplementation more cost-effectively than hitherto.

According to a first aspect of the present invention an engine pistoncomprises an outer shell, including a crown centred on a longitudinalpiston axis and a tubular side wall extending axially with respect tothe periphery of the crown to an open end and, within the tubular sidewall of the shell, a mounting member arranged to extend transversely tothe longitudinal axis and bonded both to the crown and to the side wallspaced from the crown, and gudgeon pin boss means carried by themounting member.

The term “longitudinal axis” is employed in relation to defining thepiston with respect to the geometric centre of the crown, andnotwithstanding that the cross section of the piston is other thancircular, for example, is to a small extent elliptical or oval.

Preferably, the tubular side wall includes, adjacent the crown, a regionof axially spaced, circumferentially extending ring grooves and themounting member is bonded to the side wall at the end of the ring grooveregion remote from the crown and the periphery of the mounting member isbonded to the peripheral side wall substantially at the same axialposition as at least one ring groove.

More preferably, the peripheral region the crown, the tubular side walland the bonded mounting member define therebetween an annular coolingchamber.

Preferably, the gudgeon pin boss means is provided integrally with themounting member, but notwithstanding this the bonding of the mountingmember to the crown and to the side wall displaced from the crown createa monocoque type of structure which includes the ring groove region andprovides great pressure resistance therefor without need for substantialwall thickness.

According to a second aspect of the present invention a method ofmanufacturing an engine piston comprises forming an outer shell partcomprising a crown, centred on a longitudinal axis, and a tubular sidewall, extending axially with respect to the periphery of the crown to anopen end, forming a mounting member, carrying gudgeon pin boss meansthereon, with a periphery dimensioned to fit within and interface withthe tubular side wall, disposing the mounting member within the tubularside wall such that it interfaces with the crown at a crown interfaceand interfaces with the side wall at a wall interface and bonding themounting member to the shell at said crown and wall interfaces.

Preferably, the method comprises forming the tubular side wall and atleast the peripheral part of the crown, bounding a central crown region,as a integral shell body.

Preferably the outer shell and mounting member are provided separately,as unitary or pre-assembled bodies and are then bonded metallurgicallyto form the piston.

In this specification references to bonding metallurgically are intendedto mean all known techniques employed in joining metal bodies directlyto each other or by way of an intervening metal, and includes brazingand various forms of welding, such as friction welding and laser orother beam or jet welding.

The outer shell body may be formed by back extrusion or forging.Alternatively, the outer shell body may be formed by flow forming.

If, as may be considered the norm, combustion bowl means is required inthe crown this may be formed integrally with the shell body, subject toshape constraints, or may be formed separately and metallurgicallybonded.

The outer shell part and/or mounting member may be made from steel whichis suitably ductile, but the method is equally applicable to ductilealloys of non-ferrous materials.

Embodiments of the invention will now be described by way of examplewith reference to the accompanying drawings, in which:—

FIG. 1(a) is a sectional elevation through a first embodiment of anengine piston in accordance with the present invention, taken in thedirection 1 a-1 a of FIG. 1(b), comprising an assembly of outer shellpart and mounting member bonded together,

FIG. 1(b) is a sectional elevation through the piston of FIG. 1(a) atright angles to the plane of that figure in the direction 1 b-1 bthereof,

FIGS. 1(c) and 1(d) are sectional half elevations through the outershell of FIGS. 1(a) and 1(b) respectively,

FIG. 2(a) is a sectional elevation through a second embodiment of anengine piston in accordance with the present invention, taken in thedirection 2 a-2 a of FIG. 2(b), comprising an assembly of outer shellpart and mounting member bonded together,

FIG. 2(b) is a sectional elevation through the piston of FIG. 2(a) atright angles to the plane of that figure in the direction 2 b-2 bthereof,

FIGS. 2(c) and 2(d) are sectional half elevations through the outershell of FIGS. 2(a) and 2(b) respectively,

FIG. 3(a) is a sectional elevation through a third embodiment of anengine piston in accordance with the present invention, taken in thedirection 3 a-3 a of FIG. 3(b), comprising an assembly of outer shellpart and mounting member bonded together,

FIG. 3(b) is a sectional elevation through the piston of FIG. 3(a) atright angles to the plane of that figure in the direction 3 b-3 bthereof,

FIGS. 3(c) and 3(d) are sectional half elevations through the outershell of FIGS. 3(a) and 3(b) respectively,

FIG. 4 is a schematic sectional elevation through apparatus for formingthe outer shell body by back extrusion from a slug of metal,

FIG. 5 is a schematic sectional elevation through apparatus for formingthe outer shell body by flow forming a disc of metal centred on thecrown and about the longitudinal piston axis, and

FIG. 6 is a sectional elevation through a part of a third embodiment ofpiston according to the invention in which the tubular side wall is ofsubstantially uniform thickness and the ring grooves and shoulder forthe mounting are defined by folding the metal of the wall radially as afunction of distance along the piston axis.

Referring to FIGS. 1(a) to 1(d) a piston 10 for an internal combustionengine is formed from high carbon steel. It comprises an outer shell 12and a mounting member 14 bonded to it metallurgically by brazing.

The outer shell 12 comprises a crown 16 centred on a longitudinal pistonaxis 18, and a tubular side wall 20 extending axially with respect toperipheral region 22 of the crown to an open end 24.

Centrally of the crown, and surrounded by the peripheral region 22, is acentral region 26 (denoted by boundary lines 28) in the form of acombustion bowl 30 having a bowl floor 32 displaced axially with respectto the crown peripheral region, and towards the open end of the tubularside wall, by a bowl wall 34. The bowl wall conveniently has a radiallyreentrant form as indicated at 36.

The crown 16, including both the central region 26 and peripheral region22, is of integral formation with the tubular side wall by backextrusion onto a mandrel, as described below, to define a unitary outershell body.

The tubular side wall 20 includes adjacent the crown a region 40 ofaxially spaced, circumferentially extending ring grooves 42 machinedinto the wall and between the ring groove region and the open end 24,there is provided shoulder means 44 facing towards the open end effectedby changes in internal diameter of the wall.

As best seen in FIGS. 1(c) and 1(d), the shoulder means 44 comprises afirst, smaller reduction in thickness of the ring groove region at 46,defining first shoulder 48, and as second, larger reduction in thicknessat 50 between the ring groove region and open end 24, defining a secondshoulder 52.

The tubular side wall is, apart from the ring grooves, of substantiallyuniform thickness in the groove region between the crown and shouldermeans, and also a reduced, but substantially uniform thickness betweenthe shoulder means and the open end; the reduction in thickness isprincipally defined by the shoulder means but there is also a slighttapering of wall thickness from crown to open end to minimise overallweight by having less wall thickness where less strength is required.

The region 46 may, and not disadvantageously, lie at the same axialposition as one or more of the ring grooves.

The mounting member 14 is cast by investment casting or the like andcomprises an axially thin plate 54 dimensioned to fit within the openend of the tubular side wall such that at least some points at itsperiphery 56, and preferably all of its periphery, interface with thewall shoulder region 46 at wall interface 57 and byway of whichinterface it is bonded to the outer shell. In this embodiment it isbonded to the tubular wall about substantially the whole of itsperiphery and to this end, the plate has increased axial thickness atits periphery, defining a flange 58 extending axially to one side of theplate towards the crown.

The mounting member plate 54 also carries gudgeon boss means 60 formedintegrally therewith at the side facing towards the open end 24 andaxially between the flange 58 and the open end. The gudgeon pin bossmeans includes a bore 62 for the passage of a conventional gudgeon pin(not shown) transversely to the longitudinal axis 18. The mountingmember plate 54 further includes a connecting rod space, in the form ofaperture 64 extending through the mounting member along the longitudinalpiston axis, said aperture defining from the gudgeon pin boss means twogudgeon pin bosses 66 and 68 spaced apart along the bore and, with theplate in position exposing the central region of the crown to the openend of the tubular side wall.

The central region of the crown, in particular the junction between thecombustion bowl floor 32 and wall 34, has at least one axial extensionto the bowl wall, conveniently as a circumferentially complete flange 70which provides a uniform surface extending transversely to the pistonaxis and against which the mounting member plate 54 can bear at crowninterface 71 to define its axial position within the tubular wall. Inthis embodiment, the axial position of the flange 70 is such that theperipheral flange 58 of the plate is clear of the first shoulder 48,that is, the mounting member is positioned to one axial datum only.

The mounting member is bonded metallurgically to the side wall atinterface 57 and to the crown at interface 71 by brazing, by applying abrazing material to the interfaces between the mounting member and outershell as they are assembled together, heating them to a temperaturesufficient to melt the brazing material, followed by any heat treatment,cooling and/or quenching regime desirable to impart desired physicalproperties to the brazed components. Insofar as the mounting member andouter shell are fully heated, the individual components may be subjectedto stress relieving prior to assembly and heating together.

The flange 70 which surrounds the combustion bowl floor also surroundsthe connecting rod aperture 64 such that the crown, the tubular sidewall and the bonded mounting member define therebetween an annularcooling chamber 80 which is substantially closed in the axial directionby the crown and by the mounting member. Channel means, indicatedgenerally at 82, permits passage of cooling fluid to and from theannular chamber. The channel means comprises a fluid admission aperture84 extending through the mounting member in a substantially axialdirection and disposed such that for at least part of the piston strokea jet of fluid is directed through the aperture and into the chamber. Afluid drainage aperture 86 extends through the mounting member displacedabout the longitudinal axis from the admission aperture.

In keeping with producing a light weight piston, the tubular side wallis, at the open end 24 cut away about the longitudinal piston axis inline with the ends of the gudgeon pin bore to an axial level between theends of the gudgeon pin bore and the ring groove region. It will beappreciated that the side wall may be cut more severely thanillustrated, to the level of the shoulder means or other demarcation ofthe end of the ring groove region, such that there exists, to each sideof the pin boss means two circumferentially discrete skirt portionsessentially isolated from each other.

It will be appreciated that having formed the shell 12 and mountingmember 14, there is essentially only a single assembly operation inrespect of positioning the mounting member within the outer shell andbonding it thereto, more particularly to the side wall and crowncombustion bowl, but that even with the use of relatively thin-sectionedouter shell and mounting member components, the resulting structure hasconsiderable strength and resistance to deformation of the side wall inthe ring groove region. In the same manner that a so-called monocoquestructure gives strength and stiffness to a vehicle body, the structurehere is analogous and may be considered as a monocoque type ofstructure. The strength is attributable to the structural shape as wellas the materials and the piston can thus be formed by relatively lowcost high or medium carbon steel, that is, a low alloy steel.

Indeed the construction is suitable for non-ferrous alloys provided theyare capable of being shaped into such outer shell and mounting memberand bonded together. Furthermore, it will be seen that the outer shelland mounting member may be made of different metals provided they can besuccessfully bonded or are compatible in terms of strength and thermalexpansion and ability to form a metallurgical bond between them. Theaforementioned brazing may be employed with similar or dissimilarmetals. It may be possible to effect bonding between the mounting memberand outer shell by a non-metallurgical bond. It may also be possible toprovide one or both of the outer shell and mounting member components ofnon-metallic material subject to the above criteria for bonding. Wherethe outer shell is constructed with a discrete combustion bowl or othercentral region, that part may also be non-metallic.

As mentioned above the outer shell 12 is formed as an integral body byback extrusion. Referring to FIG. 4, this shows schematically a backextrusion apparatus 400 including a mandrel 402 having an outer surface404 conforming to the internal shape and dimensions of the tubular sidewall 20, including a step 406 corresponding to shoulder means 404 and ashallow lengthwise taper that effects minimal wall thickness accordingto strength requirements, as well as recess 408 corresponding tocombustion bowl 30. A cylindrical sleeve 410 surrounds the mandrel,separated by gap 418 and relative movement between them exerts pressureon a metal slug 420 which deforms and flows into the gap and intoconformity with the mandrel to define the shell body. The aforementionedsmall taper of mandrel surface 404 that creates the above discussedinternal side wall taper also facilitates removal of the extruded shell.Such taper may be kept to a minimum, insofar as this is consistent withstrength but may be eliminated altogether without affecting removal fromthe mandrel.

It will be appreciated that such extrusion, in distinction from forging,casting and like operations, is a precision operation that permits theformation of relatively thin walls of relatively uniform thickness whichin conjunction with the strength and stiffness afforded by the monocoquetype of structure permits formation of a lightweight piston from a densematerial such as steel. However it is quite possible to provide theouter body shell by forging or other metal deforming processes.

As described hereinbefore, the combustion bowl wall has an axialextension in the form of flange 70. It will be appreciated that themounting member could interface directly with the floor of thecombustion bowl or such axial extension could be formed on the mountingmember and extend to the bowl floor. It will also be understood that thecombustion bowl may be omitted altogether, that is, have a substantiallyflat or domed crown, and such flange extend from the mounting memberplate to the bottom of the crown surface, such arrangement stillproviding the support between crown and gudgeon pin boss means andannular cooling chamber.

Although it is convenient for the central region 26 of the crown to beintegral with the peripheral region 22 it need not be, and can bemanufactured separately and welded into the peripheral region to effectthe unitary outer shell. Such separate formation of a combustion bowlmay be appropriate to avoid having to machine radial re-entrant featuresin situ, but it is, of course, not necessary for a combustion bowl tohave such re-entrant features and it may have a side wall that issuitable for forming completely by the extrusion process that definesthe shell.

Although the piston 10 is essentially constructed from two componentsbrought together in a single bonding operation, it is anticipated thatthere will still be machining operations required to the externalsurfaces of the side wall and crown, such as definition of combustionbowl re-entrant features, making valve pockets or recesses in the faceof the crown, forming of ring grooves, cutting of the wall end andapplying a final surface finish that also defines outside dimensions towithin fine tolerances. Some of these may be performed before or afterassembly and bonding of the outer shell and mounting member and some maybe achieved during the extrusion that forms the outer shell, forexample, valve pockets 90 in the crown face and ovality of cross sectiondefined by the extrusion mandrel. The degree of ovality required of apiston is usually a function of its overall diameter; it is anticipatedthat the degree of ovality required on small diameter pistons may beachieved by the final machining of the outer surface, whereas for largerdiameter pistons, such ovality may be better achieved by forming theouter shell with such cross section on a suitably shaped mandrel. Also,the formation of valve pockets or other shallow crown face features withthe shell eliminates at least one relatively costly machining operation.

Formation of the outer shell by back extrusion about a mandrel permitsthe tubular wall internal surface to be defined to a suitable degree ofaccuracy without further machining. Of particular importance inconnection with forming a lightweight piston is that in addition tobeing able to cut away any non-essential parts of the tubular side wall,is to have all wall sections as thin as possible for the functionsrequired thereof. To this end it is possible to extrude the side wallwith only slight variation in thickness from end to end (other than atthe shoulder means) and with the ring groove region also less thick thanmight be thought acceptable, because of the support from the transversemounting member.

It is a feature of the embodiment described that upon assembly prior tobonding, the interface 57 between the mounting member and the side wallextends axially, and thus positions the mounting member radially,whereas the crown interface 71 extends radially and positions themounting member axially. The bond at interface 71, which is enclosedbetween the mounting member and crown, must be effected by theaforementioned brazing or some other technique which does not rely uponaccess to it. One alternative is friction welding, but that may beconsidered unsuitable for the axially extending wall interface 57, andalthough the latter is accessible from the open end, and susceptible tobonding by a different technique, it may be preferred not to havedifferent bonding systems in use together.

This and other alternative structural possibilities are addressed in asecond exemplary embodiment of piston in accordance with the inventionis shown at 110 in FIGS. 2(a) to 2(d). Many of the parts are similar tothose of piston 10 and the description will concentrate on thedifferences, For ease of reference, corresponding parts have referencenumbers increased by 100. The piston 110 is a bonded assembly of outershell 112 and mounting member 114, piston crown including a combustionbowl 130 therein. The outer shell 112 is generally similar to shell 12insofar as it has tubular side wall 120 that includes a ring grooveregion 140 and shoulder means 144 and, integral therewith, a crownperipheral region 122. The peripheral region includes, displaced fromthe side wall, an axially extending combustion bowl wall 134. A centralregion 126 of the crown, defined by boundary lines 128 aboutlongitudinal axis 18, comprises the floor 132 of the combustion bowlwhich is of discrete formation from the peripheral region and bondedthereto at interface 133 which extends around the periphery of thiscentral region and substantially parallel to the longitudinal axis 18.

The combustion bowl floor 132 is carried by the mounting member 114,being formed integrally therewith and overlying a connecting rod space164, corresponding to the connecting rod aperture 64 of member 14 ofpiston 10, the interface 133 that the bowl floor makes with the bowlwall comprises a crown interface.

The mounting member 114 comprises a plate 154 carrying on the surfaceopposite to the combustion bowl gudgeon pin boss means 160, includingtransverse bore 162 and bosses 166 and 168 spaced apart by connectingrod space 164. The outer periphery of the member at 156 is defined to bea tight fit within the tubular side wall, in particular interfacing withthe wall region 146 at wall interface 157 which extends in asubstantially longitudinal direction.

The base of the combustion bowl wall, adjacent the floor, has axialextension 170 which abuts the upper surface of the mounting member platein order to locate it axially with respect to the crown periphery.Radial location is effected by the crown interface 133 and wallinterface 157, although the wall interface 157, insofar as it is definedat shoulder means, may provide the axial location.

It will be appreciated that the mounting member is bonded to the tubularside wall at interface 157 and to the crown at interface 133, butsignificantly, in addition to the wall interface 157 being in line withthe open end of the side wall the crown interface 133 is in line withthe open end of the combustion bowl.

As each interface is accessible in the axial direction, it is possibleto weld each by laser beam, particle beam, plasma jet or the like byrotating the piston assembly or the welding apparatus about thelongitudinal piston axis. To facilitate such a welded bond, the surfacesbounding each interface are formed to provide a small divergence in thedirection from which such welding is effected.

It will be understood that insofar as each interface is welded by aremote energy source in line therewith, the line of one or bothinterfaces may be inclined with respect to the longitudinal axis suchthat one or both of the interfaces are not only visible from without thepiston but may have a taper that effects both radial and axial locationbetween the mounting member and the outer shell. It is, of course,possible to effect bonding between the outer shell and mounting memberat the interfaces by brazing as described above.

A cooling chamber 180 is defined between the outer shell and mountingmember plate 154 and the plate 154 has fluid admission channel 184 anddrainage channel 186 therethrough. This arrangement is shown to differfrom that of piston 10 in that the chamber 180 is of greater axialextent in line with the gudgeon pin bore 162, that is, overlying thebosses 166 and 168 as shown at 167 and 169. Insofar as the admission anddrainage channels are at an operationally higher level, these extendedregions form reservoirs for cooling fluid.

In a modification to the above, the fluid drainage channel may compriseone or more channels 188 extending substantially radially from thecooling chamber to the connecting rod space 164. Such arrangement ofcooling chamber reservoirs and drainage channels may be applied to thepiston 10.

It will be appreciated that the central region 126 may be defined asbeing other than what is substantially the whole of the combustion bowlfloor. It may, for example be a smaller region of the floor or it may belarger and incorporate the bowl wall 134, the boundary between centraland peripheral regions being at the upper crown surface, as shown byboundary lines 128′, and the mounting member/crown interface 133coincident therewith, said central and peripheral regions definingtogether an essentially flat topped or domed crown.

In a further modification, not specifically shown, the central region ofthe crown may be formed with the peripheral region of the crown as partof the outer shell in the manner of piston 10 and the mounting membermay have an upstanding closure to a connecting rod space in the mannerof piston 110, whereby the closure provides not only upstanding flangemeans as described above for defining the cooling chamber but alsooverlies, and is capable of spreading load from, the central region ofthe crown.

Sectional views of a third exemplary embodiment of piston 210 inaccordance with the present invention are shown in FIGS. 3(a) to 3(d),parts corresponding to those of FIGS. 1(a) to 1(d) having referencenumbers increased by 200. The piston 210 comprises outer shell 212 andmounting member 214 bonded to each other.

The outer shell 212 comprises a unitary body consisting of crownperipheral region 222, crown central region 226 in the form of acombustion bowl 230 and tubular side wall 220. The tubular side wallconsists of a thicker ring groove region 240 adjacent the crown and athinner skirt region, open ended at 224, separated from the ring grooveregion by simple shoulder means 244. The combustion bowl 230 comprises abowl floor 232 displaced axially from the peripheral region by bowl wall234 and the wall, at the junction with the floor, has a number of axialextensions 270 ₁, 270 ₂ with gaps between them and possibly of slightlydifferent axial lengths.

Mounting member 214 comprises a relatively thin mounting plate 254, theperiphery of which is dimensioned to fit within the thinner part of theside wall adjacent the shoulder means 244; the periphery 256 of theplate defines an interface 257 with the tubular wall and has no axialflange or like projection to increase the axial length of the interface.

The lower face of the plate, facing the open end 224, caries integralgudgeon pin boss means 260 having transverse gudgeon pin bore 262therethrough and through the plate, along piston axis 18, is aconnecting rod aperture 264 which also effects formation of separatedgudgeon pin bosses 266 and 268.

The upper face of the plate, indicated at 255 and facing the crown, issubstantially flat and abuts the bowl wall extensions 270, and 2702defining thereat interface 271 extending transversely with respect tothe piston axis 18. Insofar as the interfaces 257 and 271 correspond inposition and orientation to the interfaces 57 and 71 of piston 10, theouter shell and mounting member in the disposition shown are bonded toeach other by brazing as described above, defining the strong monocoquetype of structure including a closed annular cooling chamber 280 betweenthe crown, ring groove region of the side wall and the upper surface 255of the mounting member plate 254. Fluid channel means 284 permitsadmission of cooling fluid into the chamber and channels 288 permitdrainage by way of the gaps between the bowl wall axial extensions 270,and 2702 to the connecting rod aperture.

As an alternative to the equally-applicable cutting away of the open endof the side wall to the extent shown for pistons 10 and 110, the gudgeonpin bosses 266 and 268 may extend into abutment with the side wall andthe latter include through apertures 290 and 292 in alignment with thegudgeon pin bore 262. Furthermore, insofar as each boss defines aninterface 294, 296 respectively with the side wall, it may be bondedthereto adjacent the apertures.

This arrangement of longer, apertured side wall and, optionally, gudgeonpin bosses extending thereto may be applied to the piston 10, and topiston 110 provided that the mounting member is bonded by brazing or thelike that does not require direct access.

The mounting means 214 is of substantially uniform cross section in thedirection of the gudgeon pin bore 262, that is, as viewed in FIG. 3(a),except of course where the connecting rod aperture 264 is cut. Insteadof the mounting member being cast, it is formed by cutting from anextruded stock and then shaped to fit within the tubular side wall andthe axially extending apertures 264 and 284 cut therein.

It will be appreciated that a cast mounting member, having a morecomplex surface as seen in the above described embodiments may beemployed in piston 210 or such an extruded mounting member may beemployed with the pistons 10 and 110.

Another difference illustrated in this embodiment is a tubular side wallwhich is, to each side of the shoulder means 244, of uniform thickness,that is, without the normally slightly tapering characteristic ofextrusion or forging. The outer shell 212, although it may be formed byextrusion or forging, is produced by so-called flow forming in which, asFIG. 5 illustrates a disc-like slug of metal 502 is caused to rotatewith a profiled mandrel 504 and during rotation the peripheral regionsof the disc are displaced axially to lie along and conform with themandrel that defines the respective wall thicknesses and shoulder means.As with the back extrusion described above, the tubular side wall may beformed thereon and removed without an internal wall taper, where thiscan save weight.

Such flow forming of the outer shell may be used in respect of piston 10and 110. Although it may be preferred to define the tubular side wallwith a thicker region into which ring grooves are subsequently machined,it will be appreciated that such flow forming permits, with the use of aradially contractible mandrel, formation of a tubular side wall 220′ ofsubstantially uniform thickness from end to end but varying in radius asa function of axial position to define the ring grooves 242′ andshoulder means 244′, as illustrated in FIG. 6.

It will also be appreciated that whereas the above embodiments havedescribed the mounting member as bonded about the whole of its peripheryto the side wall at the shoulder means, it may be bonded only at aplurality of discrete points and the periphery of the mounting membermay be other than conforming in shape to the tubular side wall,extending to contact the wall only at points of bonding.

Although it is convenient for assembly to define a bonding interfacebetween the periphery of the mounting member to the side wall atshoulder means which demarcates between the ring groove region and themore lightly loaded skirt, it is not essential and it may be bonded tothe side wall other than at such shoulder if disposing the interfaceelsewhere improves stress patterns.

It is re-iterated that the various embodiments of piston according tothe invention are not limited in size and the structures and methods ofmanufacture are capable of being scaled to a wide variety of dimensions.

1-72. (canceled)
 73. An engine piston comprising a prefabricated outer shell, said shell including a crown centred on a longitudinal piston axis and a tubular side wall extending axially with respect to the periphery of the crown to an open end; and a prefabricated mounting member within the tubular side wall of the shell, said mounting member carrying gudgeon pin bosses and extending transversely to the longitudinal piston axis, said mounting member also having an upper member part interfacing with the crown at a radially extending crown location interface between them serving as a mutual axial location reference and interfacing with the side wall spaced from the crown, defining a transversely extending closure plate-like member closing partly from below a peripheral chamber in the crown, the mounting member being bonded permanently to the crown at a crown bonding interface and bonded permanently to the side wall by at least one axially extending peripheral interface with the side wall.
 74. A piston according to claim 73 wherein the bond between the mounting member and the tubular side wall is at substantially the same longitudinal position as the bond between the mounting member and the crown.
 75. A piston according to claim 73 wherein the tubular side wall includes, adjacent the crown, a region of axially spaced, circumferentially extending ring grooves and the mounting member is bonded to the side wall at the end of the ring groove region remote from the crown.
 76. A piston according to claim 73 wherein the crown has a central region surrounded by a peripheral region, the peripheral region being of integral formation with the tubular side wall, and the central region comprises a combustion bowl having a bowl wall and a bowl floor displaced axially from the crown peripheral region by said bowl wall and the bowl floor being formed by the mounting member bonded to the bowl wall.
 77. A piston according to claim 76 wherein the combustion bowl floor and bowl wall are bonded together at a bonding interface between them extending in a substantially axial direction.
 78. A piston according to claim 77 wherein the bonding interface is accessible from externally of the combustion bowl.
 79. A piston according to claim 76 wherein said radially extending interface between the mounting member and crown serving as a mutual axial location reference is disposed between the mounting member and an end region of the bowl wall.
 80. A piston according to claim 76 wherein the mounting member includes at least one fluid channel between the peripheral chamber and shell open end, at least one said channel extending through the mounting member from a said upper part defining the peripheral chamber to below the combustion bowl floor defined by the mounting member.
 81. A piston as claimed in claim 76 wherein the gudgeon pin bosses are defined integrally with, and provided solely by, the mounting member on a face of the mounting member facing the open end of the tubular wall member and disposed axially between the open end of the shell and the bond between the mounting member and side wall, said bosses including a bore for the passage of a gudgeon pin extending transversely to the longitudinal piston axis and being spaced apart along the axis of said bore the gudgeon pin bosses.
 82. A piston according to claim 73 in which the crown has a central region surrounded by a peripheral region, the peripheral region being of integral formation with the tubular side wall and the central region comprises a combustion bowl having a bowl wall and a bowl floor displaced axially from the crown peripheral region by said bowl wall, the combustion bowl floor being formed integrally with the bowl wall and said mounting member being bonded to the crown at the combustion bowl.
 83. A piston according to claim 82 wherein said radially extending interface between the mounting member and crown serving as a mutual axial location reference is defined between the mounting member and the crown at the junction between the combustion bowl wall and bowl floor.
 84. A piston according to claim 82 wherein said radially extending interface is defined by an axial extension to the combustion bowl wall.
 85. A piston according to claim 82 wherein the mounting member is bonded to the crown at said radially extending crown location interface between the mounting member and crown serving as a mutual axial location reference.
 86. A piston as claimed in claim 82 wherein the gudgeon pin bosses are defined integrally with, and provided solely by, the mounting member on a face of the mounting member facing the open end of the tubular wall member and axially between the open end of the shell and the bond between the mounting member and side wall, said bosses including a bore for the passage of a gudgeon pin extending transversely to the longitudinal piston axis and being spaced apart along the axis of said bore the gudgeon pin bosses.
 87. A piston as claimed in claim 86 wherein the mounting member comprises an axially thin plate having increased axial thickness at the periphery and to effect the piston bosses.
 88. A piston as claimed in claim 86 wherein the mounting member includes a connecting rod aperture extending through the mounting member along the longitudinal piston axis between said gudgeon pin bosses and exposing the central region of the crown to the open end of the tubular side wall.
 89. A piston as claimed in claim 88 wherein the mounting member is of substantially uniform cross-section in said direction of the gudgeon pin bore, except for the connecting rod aperture.
 90. A piston as claimed in claim 88 wherein the mounting member upper part has an upper surface and includes at least one fluid channel between the peripheral chamber and shell open end, at least one said fluid channel extending in the upper surface of the mounting plate from said peripheral chamber to the connecting rod aperture.
 91. A piston as claimed in claim 73 wherein the junction between the periphery of the mounting member and the shoulder means is in line with the open end of the tubular side wall.
 92. A piston as claimed in claim 73 wherein the gudgeon pin boss means includes a bore for the passage of a gudgeon pin transversely to the longitudinal piston axis and the side wall includes cooperating apertures in the tubular side wall and the boss means is bonded with the tubular side wall at said apertures.
 93. A piston as claimed in claim 73 wherein the tubular side wall has therein shoulder means provided by a change in internal diameter of the wall and facing towards the open end of the side wall, and the periphery of the mounting member is bonded to the side wall at the shoulder means.
 94. A piston as claimed in claim 88 wherein the tubular side wall has between the end of the ring groove section and said open end a reduction in thickness defined principally by the shoulder means, the shoulder means comprises a first, smaller thickness of the ring groove region and a second, larger reduction in thickness between the ring groove region and open end.
 95. A piston as claimed in claim 73 wherein the outer shell is made from a metal alloy deformable in the solid state.
 96. A piston as claimed in claim 95 wherein the material is steel.
 97. A piston as claimed in claim 95 wherein the mounting member and outer shell are made from the same material.
 98. A piston as claimed in claim 95 wherein the mounting member and outer shell are bonded together by at least one welded joint.
 99. A method of making an engine piston comprising forming an outer shell part comprising a crown, centered on a longitudinal axis, and a tubular side wall extending axially with respect to the periphery of the crown to an open end, said crown having at least a peripheral region integral with the side wall; forming a mounting member having an upper member part and opposite thereto gudgeon pin bosses and a periphery dimensioned to fit within and interface with the tubular side wall; disposing the mounting member within the tubular side wall such that the gudgeon pin bosses face the open end, the upper part interfaces with the crown at least a radially extending crown interface defining mutual axial location reference, and the periphery interfaces with the side wall at a wall interface spaced axially from the crown periphery and radially from said radially extending crown interface, defining thereby a transversely extending closure plate closing partly from below a peripheral chamber in the crown; and permanently bonding the mounting member to the shell part at said wall interface and to the crown at a crown bonding interface radially inwardly of said wall interface.
 100. The method of claim 99 comprising bonding the mounting member to said crown and wall at interfaces at substantially the same axial position.
 101. The method of claim 99 comprising forming in the tubular side wall adjacent the crown a region of axially spaced circumferentially extending ring grooves and bonding the periphery of the mounting member to the side wall at the end of the ring groove region remote from the crown.
 102. The method of claim 99 comprising forming the tubular side wall and at least a peripheral region of the crown, bounding a central crown region, as an integral shell body and by forming at least part of the central crown region as a combustion bowl having a bowl wall and a bowl floor displaced axially of the crown peripheral region by said bowl wall, and by bonding the mounting member to the crown at the combustion bowl.
 103. The method of claim 102 comprising defining said radially extending crown location interface between the mounting member and crown, serving as a mutual axial location reference between the mounting member and the crown, at the junction between the combustion bowl wall and bowl floor.
 104. The method according to claim 103 comprising defining said radially extending interface by an axial extension to the combustion bowl wall.
 105. The method according to claim 102 comprising bonding the mounting member to the crown at the radially extending crown location interface between the mounting member and crown serving as a mutual axial location reference.
 106. The method of claim 99 comprising forming the tubular side wall and at least a peripheral region of the crown, bounding a central crown region, as an integral shell body and by forming at least part of the central crown region as a combustion bowl having a bowl wall and defining a bowl floor displaced axially of the crown peripheral region by said bowl wall by the mounting member.
 107. The method of claim 106 comprising defining said radially extending crown location interface, serving as a mutual axial location reference, between the mounting member and the combustion bowl wall.
 108. The method of claim 106 comprising bonding the mounting member to the crown by way of a said crown bonding interface between the combustion bowl floor defined by the mounting member and the combustion bowl wall defined by the crown.
 109. The method of claim 108 comprising defining said crown bonding interface extending in a substantially axial direction accessible from outside of the combustion bowl and bonding the combustion bowl floor to the combustion bowl wall by access through the combustion bowl.
 110. The method of claim 109 comprising bonding the combustion bowl floor to the combustion bowl wall metallurgically by applying heat from a source externally of the piston by access through the combustion bowl.
 111. The method of claim 110 comprising bonding the interface by welding.
 112. The method of claim 99 comprising defining the wall interface accessible from externally of the piston and bonding metallurgically by applying heat from a source externally of the piston.
 113. The method of claim 112 comprising bonding the interface by welding.
 114. The method of claim 99 comprising bonding the mounting member to the side wall substantially about the whole of its periphery.
 115. The method of claim 99 comprising casting the mounting member.
 116. The method of claim 99 comprising extruding the mounting member.
 117. The method of claim 99 comprising forming the outer shell part by flow forming about a rotating mandrel shaped to define the interior of the tubular side wall.
 118. The method of claim 99 comprising forming the outer shell part by back extrusion about a mandrel shaped to define the interior of the tubular side wall.
 119. The method of claim 118 comprising forming at least one valve pocket in the crown during extrusion.
 120. The method of claim 99 comprising forming the body shell and mounting member from the same material.
 121. The method of claim 99 comprising forming the outer shell part from steel.
 122. A method of making an engine piston having a crown, centered on a longitudinal axis, and a tubular side wall, extending axially with respect to the periphery of the crown to an open end and defining a region for a belt of ring grooves and a skirt, the method being characterized by rotating a unitary body of metal and by flow forming thereof defining at least a peripheral portion of piston crown and the side wall extending therefrom.
 123. A method of making an engine piston having a crown, centered on a longitudinal axis, and a tubular side wall, extending axially with respect to the periphery of the crown to an open end and defining a region for a belt of ring grooves and a skirt, the method being characterized by back extruding a unitary body of metal to define at least a peripheral portion of piston crown and the side wall extending therefrom. 